Thin Film Field Effect Transistor with Dual Semiconductor Layers

ABSTRACT

A thin film field effect transistor is disclosed which provides improved time-based channel stability. The field effect transistor includes first and second disordered semiconductor layers separated by an insulator. In an embodiment a carrier injection terminal is provided in a thin semiconductor layer closest to the gate terminal. An electric field is established in the thin semiconductor layer. At sufficient field strength, the electric field extends into the second semiconductor layer, which is in contact with the source and drain terminals. At sufficient field strength a channel is established in the second semiconductor layer, permitting current to flow between source and drain terminals. Above a certain gate voltage, there is sufficient free charge is induced in the first semiconductor layer so that the field does not extend into the second semiconductor, effectively shutting off current between source and drain. Single-device transition detection (as well as other applications) may be obtained.

BACKGROUND

The present disclosure is related to thin film transistors, and morespecifically to a thin film field effect transistor structure with dualsemiconductor and insulator layers providing improved device stabilityand performance.

A field effect transistor (FET) is an electronic device in which currentis permitted to flow between a source and a drain terminal in aconductive region, or channel, created by a voltage bias establishedacross a gate terminal and the source terminal. Varying the voltage atthe gate varies the channel size and conductivity, allowing the gate tocontrol the flow of current between source and drain.

A thin film transistor (TFT) is typically a FET which is formed over aninsulating substrate and in which the source, drain, and gate, as wellas the layer in which the channel is established are formed from thinlayers of material. The substrate is often glass, ceramic, plastic,fiberglass, or similar insulating material. The source, drain, and gateterminals are often formed of a patterned metal. The channel layer istypically a semiconductor, and commonly silicon (Si). Certaincharacteristics of the TFT are determined by the crystalline state ofthe semiconductor layer, which may be single crystal Si, polycrystallineSi, or amorphous Si. As used herein, polycrystalline, re-crystallized,amorphous materials, and suspension and polymer materials arecollectively referred to as disordered materials.

Thin film FET designs and characteristics are well known. Thin film FETsare generally combined with other thin film devices to form operationalcircuits. For example, several thin film FETs may be interconnected toform switching circuits for display or detector panels, peak detectionor transition detection circuits, clock doubling circuits, etc.

In the case of amorphous, polymer, and similar disordered semiconductorapplications, individual thin film FETs exhibit an undesirableinstability during operation. For example, the semiconductor layerpresents carrier traps which fill as a function of time and current. Asthe number of positively charged carriers increases at the gate terminaldue to the application of a gate voltage, the number of negativecarriers trapped in the channel region increases. However, as a stable,fixed voltage continues to be applied to the gate terminal, the numberof negative carriers trapped in the channel increases with time. Thistime-based trapping means that even for a fixed gate voltage, thechannel current characteristics change over a relatively short period oftime. This means that in a wide variety of FET devices, such as acircuit in which a signal should issue when a fixed threshold of chargeis detected or exceeded, the threshold will in fact drift over time.

Another way to view this problem is to plot the electric field strengthin the semiconductor layer against gate voltage at several differenttimes, as shown in FIG. 1. At time t₁, a certain gate voltage v producesa certain field strength ε₁. However, at a later time t₂, that sameapplied gate voltage v produces a field strength ε2 reduced by an amountΔε as compared to the field produced by that voltage at time t₁. Thischange is field strength over time reduces carrier mobility in thechannel, and hence reduces the current sensed at the drain in responseto the gate voltage.

There have been numerous efforts to address this problem oftime-instability. One such effort involves developing circuits withnumerous thin film FETs such that instability is effectively cancelledout among the FETs. One problem with this approach is that additionalFETs increase the size and power demands of the device circuitry,contrary to the desired objective of reducing device size and powerconsumption. Another problem is that as the number of FETs increases, sodoes the likelihood that one or more of the devices will fail and renderthe circuit inaccurate or inoperative. Still further, each FET has itsown non-linearity characteristics, meaning accurate device performanceis difficult to predict. Therefore, there is a need in the art for adisordered material-based thin film FET with reduced instability, whichcan be manufactured without significantly changing manufacturingtechniques, without requiring the use of unorthodox materials, and whichcan otherwise easily be adapted to form an element of a known circuit.

SUMMARY

Accordingly, the present disclosure is directed to a thin film FEThaving improved time-based channel stability and hence improvedswitching stability and performance. Devices able to take advantage ofreduced threshold voltage shift are therefore enabled.

According to one aspect of the disclosure a thin film FET is formed ofamorphous silicon or similar disordered semiconducting material. Thethin film FET comprises a gate metal formed and patterned over aninsulating substrate. A first (shielding) insulating layer substantiallycovers the gate metal and substrate. A metal carrier injection terminalis located over the first insulating layer. A first (shielding)semiconductor layer covers the carrier injection terminal and firstinsulating layer. A second insulating layer covers the firstsemiconductor layer. Source and drain terminals are disposed over thesecond insulating layer and within a second semiconductor layer formedover the second insulating layer. Optionally, doped regions can beformed under the source and drain terminals to improve contactresistance, but such features are not strictly required.

According to another aspect of the disclosure, the thin film FET may bea bottom gate or top gate device. Furthermore, a second metal carrierinjection terminal may be provided over the first insulating layer andwithin the first semiconductor layer. In addition, the source and drainterminals may be formed over the second semiconductor region (again,with or without optional doped regions thereunder). Still further, themetal carrier injection terminal may be formed and patterned over thefirst semiconductor layer.

In operation, a field is created by a voltage at the gate terminal. Thisfield extends into the first semiconductor layer, but only very weaklyextends into the second semiconductor layer if at all. Oppositelycharged carriers, for example negative charge carriers, are trapped atthe interface between the first insulating layer and the firstsemiconductor layer, creating an excess of positive charge carriers inthe first semiconductor layer at the interface of that layer and thefirst insulating layer. Carriers for this accumulation are provided bythe carrier injection terminal. The net imbalance in carrier charge isseen by the second semiconductor layer at its interface with the secondinsulating layer. The second semiconductor essentially sees a fieldcreated in the first semiconductor layer. However, this field isstabilized in the first semiconductor layer. Thus, a much more stablechannel is created in the second semiconductor layer, and a more stabledetector may be provided without increasing device count of the finalcircuit.

The above is a summary of a number of the unique aspects, features, andadvantages of the present disclosure. However, this summary is notexhaustive. Thus, these and other aspects, features, and advantages ofthe present disclosure will become more apparent from the followingdetailed description and the appended drawings, when considered in lightof the claims provided herein.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings appended hereto like reference numerals denote likeelements between the various drawings. While illustrative, the drawingsare not drawn to scale. In the drawings:

FIG. 1 is a plot of the electric field strength against gate voltage atseveral different times for a semiconductor layer of a prior art fieldeffect transistor.

FIG. 2 is a cut-away, elevation view of a bottom gate thin film fieldeffect transistor (TFT) with dual semiconductor layers according to oneembodiment of the present disclosure.

FIGS. 3A through 3C are plots of the electric field within a first andsecond semiconductor layers against vertical position for the cases ofV_(gs)−V_(fb)=0, 0<V_(gs)−V_(fb)<V_(T), and V_(gs)−V_(fb)>>V_(T), for athin film FET according to an embodiment of the present disclosure.

FIG. 4 is a plot of output current (between source and drain) of asecond semiconductor layer as a function of the gate voltage for a thinfilm FET according to an embodiment of the present disclosure.

FIGS. 5A and 5B are graphs of source-drain current against gate voltagefor first semiconductor thicknesses of 500 Å and 300 Å, respectively,for a thin film FET according to an embodiment of the presentdisclosure.

FIG. 6 is a is a cut-away, elevation view of a top-gate thin film fieldeffect transistor (TFT) with dual semiconductor layers according to oneembodiment of the present disclosure.

FIG. 7 is a is a cut-away, elevation view of a thin film field effecttransistor (TFT) with dual semiconductor layers and dual carrierinjection terminals according to one embodiment of the presentdisclosure.

FIG. 8 is a cut-away, elevation view of a bottom gate thin film fieldeffect transistor (TFT) with dual semiconductor layers according toanother embodiment of the present disclosure.

FIG. 9 is a schematic representation of a transition detection deviceaccording to an embodiment of the present disclosure.

DETAILED DESCRIPTION

We initially point out that descriptions of well known startingmaterials, processing techniques, components, equipment and other wellknown details are merely summarized or are omitted so as not tounnecessarily obscure the details of the present disclosure. Thus, wheredetails are otherwise well known, we leave it to the application of thepresent disclosure to suggest or dictate choices relating to thosedetails.

With reference to FIG. 2, there is shown therein a thin film fieldeffect transistor (TFT) with dual semiconductor layers 10 according toone embodiment of the present disclosure. Thin film TFT 10 comprisessubstrate 12, such as glass, ceramic, plastic, fiberglass, or similarinsulating material. A gate terminal 14 is formed on a surface ofsubstrate 12. In one embodiment, gate terminal is amorphous silicondeposited by plasma enhanced chemical vapor deposition (PECVD). Othermaterials and methods are equally useful. For example polymers can bedeposited by spincoating. As an alternative, gate terminal 14 may be adoped region of substrate 12, where substrate 12 is a conductor, such asa semiconductive silicon (Si) wafer. A first (shielding) insulator layeris next formed over gate terminal 14 and any exposed portions of thesurface of substrate 12. First insulator layer 16 may be any insulatingmaterial compatible with the semiconductor, for example silicon nitridewith amorphous silicon, deposited by a known method, such asevaporation, printing, etc., to a thickness of roughly 1000 Å. Carrierinjection terminal 18 is then formed over the surface of first insulatorlayer 16. Terminal 18 is a high work function conductive materialcapable of providing ohmic contact to a subsequently depositedsemiconductor layer, sized such that terminal 18 does not overlap thechannel region formed by source and drain metal. A first (shielding)semiconductor layer 20 is then formed over carrier injection terminal 18and the exposed portion of first insulator layer 16. First semiconductorlayer 20 is, in one embodiment, amorphous silicon, deposited to athickness of between 300 Å and 500 Å by CVD or any other technique.

Next, second insulator layer 22 is formed over first semiconductor layer20. Second insulator layer 22 may be the same material as, or differentfrom, the first insulator layer to a thickness of roughly 1000 Å. Sourceterminal 24 and drain terminal 26 are formed of a highly conductivematerial over a surface of second insulator layer 22. Finally, secondsemiconductor layer 28 is formed over source and drain terminals 24, 26and the exposed portion of second insulator layer 22. Secondsemiconductor layer 28 is, in one embodiment, amorphous silicon,deposited to a thickness of roughly 500 Å. An encapsulation layer (notshown) may optionally be formed over gate terminal second semiconductorlayer 28.

In one mode of operation both first and second semiconductor layers 20,28 are comprised of the same material. Carrier injection terminal 18,which serves in all embodiments to provide a source of carriers in firstsemiconductor layer 20, is tied to reference voltage V_(ref), such assystem ground. Source terminal 24 is tied to source voltage V_(s), whichin this case is also system ground. Gate terminal 14 and drain terminal26 are tied to gate voltage V_(g) and drain voltage V_(d), respectively.In this mode, the device disclosed above operates in several regimes.

Case 1: V_(gs)−V_(fb)=0: When the gate-source bias v_(gs) is zero, noelectric field is present in either first or second semiconductor layers20, 28. With regard to current between the source and drain, since thereare no mobile carriers in the second semiconductor layer 28 (no channelis established), the application of a non-zero bias between source anddrain, V_(ds), results only in leakage current.

Case 2: 0<V_(gs)−V_(fb)<V_(T): When a gate-source bias is applied suchthat the gate bias is less than the threshold voltage V_(T) of firstsemiconductor layer 20, there is a small amount of band bending in firstsemiconductor 20. We refer to this case as below threshold operation.

Disordered semiconductors have a large number of states in the band gap.In below threshold operation, the Fermi level lies in the deep states,the number of induced free carriers is low and the electrostatics isdefined by the localized deep state charge. The Debye length (thedistance over which effective charge separation occurs) related to thedeep states can be large. For example, in amorphous silicon (a-Si:H) itis about 540 nm. This length is not bias dependent. It is a function ofthe material and particularly the temperature at which the material wasdeposited. In general, the more disordered the material, the thicker thematerial layer may be. If first semiconductor layer 20 is made thinnerthan this, non-zero band bending is induced (i.e., an electric filed isestablished) in second semiconductor layer 28. In such a case, aconductive channel may be established in second semiconductor layer 28,and the application of a non-zero V_(ds) results in a source-draincurrent in second semiconductor layer 28.

The fact that first semiconductor layer 20 is formed of a disorderedmaterial is important. It is noted that the source-drain current peakswhen the gate voltage is between the off voltage and the channelcreation (or threshold) voltage of first semiconductor layer 20.Therefore, the device effectively works in a sub-threshold region. Inthe sub-threshold region, the Fermi level of the bottom semiconductor isdeep in the gap where the gap state density is large. For as long as theFermi level stays in this region of the band gap, there will be asignificant electric field seen by the second semiconductor layer 28. Iffirst semiconductor layer 20 was not disordered, the Fermi level wouldnot be pinned in the band gap for the relatively wide range of inputvoltages. Therefore, we have taken advantage of the disordered nature ofthe semiconductor and more particularly its structural consequence.

Note that the threshold voltage V_(T) is dependent on the thickness ofthe semiconductor for disordered semiconductors.

Case 3: V_(gs)−V_(fb)>>V_(T): When the gate bias is increased beyondV_(T), the band-bending in first semiconductor layer 20 is very sharpand most of the carriers are present at the interface between firstsemiconductor layer 20 and first insulator layer 16. The bands flattenout towards second semiconductor layer 28 and the electric field doesnot significantly permeate into second semiconductor layer 28.Therefore, since the second semiconductor layer 28 is in flat band(i.e., no field is present to induce a channel), the application of anon-zero V_(ds) results in no current flow between source and drain.

With reference to FIGS. 3A through 3C, there is shown therein plots ofthe electric field within the first and second semiconductor layers 20,28 against vertical position of the aforementioned device in each of thethree cases discussed above. In FIG. 3A corresponding to case 1 it canbe seen that there is no field established in either first or secondsemiconductor layers 20, 28. This case is below threshold voltage, sothis response is expected. In FIG. 3B corresponding to case 2 it can beseen that even though the bias is below the threshold voltage for secondsemiconductor layer 28 a field is established and current flows thereinnonetheless. This is explained by the fact that the field induced infirst semiconductor layer 20 induces a field in second semiconductorlayer 28. This field is sufficient to establish a channel and permitcarriers to flow. However, first semiconductor layer 20 effectivelyshields second semiconductor layer 28 from the interface effects whichresult in carrier trapping. Thus, first semiconductor layer 20 is alsoreferred to herein as a shielding semiconductor layer (and likewise,first insulator layer 16 is also referred to as a shielding insulatorlayer). In FIG. 3C corresponding to case 3 it can be seen that theincrease in V_(gs) results in a shallow field in first semiconductorlayer 20, due at least in part to carrier trapping. This field does notextend into or influence second semiconductor layer 28. Thus, no channelis established in second semiconductor layer 28, and the currenttherethrough is zero.

With reference to FIG. 4, there is shown therein a plot of outputcurrent (between source and drain) i_(out) of the second semiconductorlayer 28 as a function of the gate-source voltage V_(gs). For the deviceillustrated in FIG. 4, first and second insulator layers 16, 22 are each1000 Å thick, first semiconductor layer 20 is 500 Å thick, and secondsemiconductor layer 22 is 1000 Å thick. The source-drain bias was set at5V. When carrier injection terminal 18 is open, the device behaves likea poor transistor due to the effectively very thick insulating layerbetween second semiconductor layer 28 and gate terminal 14. Moreover,the current is limited by contact resistance.

Next we consider the case of carrier injection terminal 18 beingconnected to ground. When the gate (input) voltage is zero, there is novertical electric field in the device (assuming zero flatband). There isno current through second semiconductor layer 28 between source anddrain terminals 24, 26. As the input voltage is increased, the bandbending in first semiconductor layer 20 extends into secondsemiconductor layer 28. This electric field seen at second semiconductorlayer 28 results in a small induced charge, and there is an increase incurrent between source and drain terminals 24, 26. As the input voltageis increased further, there is sufficient induced free charge in firstsemiconductor layer 20 so as to completely shield second semiconductorlayer 28. Therefore, the current through second semiconductor layer 28is again small. Thus, the device effectively converts a transition ininput voltage, to a current pulse.

With reference to FIG. 4, it can be seen that unlike capacitive couplingthe direction of the current is the same for both increasing V_(gs) anddecreasing V_(gs). (The current is positive for both increasing anddecreasing V_(gs).) If the gate voltage is reduced below −1 in FIG. 4,the current will turn off until a point when the voltage is so low thatp-type conduction begins to occur. This will cause a tiny increase incurrent.

As the thickness of first semiconductor layer 20 increases the electricfield permeating into second semiconductor layer 28 weakens, ultimatelyto the point that no field enters second semiconductor layer 28.Accordingly, as the first semiconductor layer thickness increases, thesource-drain current decreases up to the point at which no current flowsat all. Conversely, as first semiconductor layer 20 decreases inthickness more of the electric field formed in first semiconductor layer20 permeates second semiconductor layer 28. The second semiconductorlayer 28 then becomes conductive at a lower V_(gs), and non-conductiveat a high V_(gs). A larger current can be supported in the channelinduced into second semiconductor layer 28. This is illustrated in FIGS.5A and 5B, which are graphs of source-drain current against gate voltagefor first semiconductor thicknesses of 500 Å and 300 Å, respectively.However, if first semiconductor layer 20 is made too thin, deviceoperation reverts to the case of a single semiconductor layer, and theissues of trapping and threshold drift reappear, and at a lower limitbelow roughly 100 Å, first semiconductor layer 20 looses uniformity andceases to be able to support an electric field. A range of thickness forfirst semiconductor layer 20 providing desirable device performance isbetween 200 Å and 1,000 Å, but thicknesses outside this range may bedesirable depending on the material system, operating conditions, etc.Thus, the lower semiconductor must be thinner than the effective Debyelength of the deep states of the material used (for a-Si, about 500 nm),yet thick enough to have a continuous film deposit without pin holes.

There is also a generally desirable ratio of the thickness of the secondsemiconductor layer 28 to first semiconductor layer 20. In oneembodiment, second semiconductor layer 28 is generally between 1.5 and2.5 times the thickness of first semiconductor layer 20. However, incertain material systems, thickness ratios outside of this range may bedesirable.

While the thickness of first semiconductor layer 20 is important to theoverall device performance, the thickness of second semiconductor layer28 is much less critical. Below a certain thickness the layer may not beable to support a carrier channel and may become highly resistive, andabove a certain thickness bulk leakage current in the semiconductor canswamp the signal current and degrade the signal to noise performance.

The behavior of device 10, as illustrated in FIGS. 3A through 3C andFIG. 4 suggest that it is well suited to a number of applications. Forexample, since i_(out) reaches a relative peak only at a point oftransition of the gate bias, the device is well suited to perform as asingle transistor threshold detection circuit. In such an application,the device is free of the problem of carrier trapping and thresholddrift observed in presently known devices. The threshold voltage shiftis proportional to V_(gs)−V_(t), where V_(t) is the threshold voltage.For 0<V_(gs)<V_(t), the threshold voltage shift is negligible.

Importantly, we have discovered that the device design disclosed herein,and variations thereon, mean that second semiconductor layer 28 is freeof excessive charge trapping due to applied bias. Thus, the devicedemonstrates a relatively much more constant threshold voltage over timeas compared with prior devices. Furthermore, while first semiconductorlayer 20 may experience a time-dependent threshold voltage shift, theincrease in threshold voltage actually increases the stability of secondsemiconductor layer 28 as its operation is effectively over a largergate voltage range.

Furthermore, while preferred exemplary embodiments have been presentedin the foregoing detailed description, it should be understood that avast number of variations exist, and these preferred exemplaryembodiments are merely representative examples, and are not intended tolimit the scope, applicability or configuration of the disclosure in anyway. For example, while the previous descriptions have focused onamorphous silicon, many other disordered inorganic semiconductormaterials as well as organic semiconductors may form the active layersof a device of the type described herein. Many different fabricationtechniques may be employed, from those described above to solutionprocessing such as ink-jet printing, spin coating, etc.

While the device described above is a bottom-gate FET, this disclosureis equally applicable to top-gate devices. Such a top-gate device 50 isshown in FIG. 6. Device 50 comprises a substrate 52 on which may beformed an optional adhesion/insulator layer 54. Formed over layer 54 aresource and drain terminals 56, 58. First semiconductor layer 60 isformed over source and drain contacts 56, 58 and any exposed portions ofthe layer or substrate formed thereunder. First insulator layer 62 isformed over first semiconductor layer 60. Carrier injection terminal 64is formed over a portion of the surface of first insulator layer 62.Second semiconductor layer 66 is formed over carrier injection terminal64 and the exposed portion of first insulator layer 62. Second insulatorlayer 68 is formed over second semiconductor layer 66. Finally, gateterminal 70 is formed over a portion of second insulator layer 68. Anencapsulation layer (not shown) may optionally be formed over gateterminal 70 and any exposed portion of second insulator layer 68. Devicefabrication steps, parameters, and device dimensions may besubstantially as described above.

Further still, a single carrier injection terminal 18 has been describedabove. However, in one variation a second carrier injection terminal 32may be employed. An embodiment of a two-carrier injection terminaldevice 30 is illustrated in FIG. 7. Device 30 is substantially the same,as is its method of manufacture, as device 10 of FIG. 2, with theexception of the formation of second carrier injection terminal 32 inthe same layer as and adjacent the first carrier injection terminal 18.The role of carrier injection terminal 32 is likewise the same as firstcarrier injection terminal 18, which is to provide carriers in responseto an electric field established in first semiconductor layer 20.

In addition, source and drain terminals 24, 26 have been formed oversecond insulator layer 22 and within second semiconductor layer 28.However, these terminals may be formed over second semiconductor layer28 as shown in embodiment 80 of FIG. 8. Optionally, implantation regions82, 84 may be formed prior to the formation of source and drainterminals 24, 26. While not strictly required, regions 82, 84 may helpreduce contact resistance.

Indeed, various presently unforeseen or unanticipated alternatives,modifications variations, or improvements therein or thereon may besubsequently made by those skilled in the art which are also intended tobe encompassed by the claims, below.

With reference now to FIG. 9, there is shown therein a schematicrepresentation of a transition detection device 100 according to anembodiment of the present disclosure. Circuit 100 comprises a thin filmFET of a type previously disclosed, such as FET 10, 30, 50 or 80. Forillustration purposes, we focus on the FET 10, connected as follows.Source terminal 24 is electrically interconnected to carrier injectionterminal 18, which are each in turn electrically interconnected to asystem ground reference potential. Gate terminal 14 is connected to adevice under test 102 (DUT) such that an output signal of the DUT drivesthe gate potential. Drain terminal 26 is then monitored for currentindicative of a threshold change in the output of the DUT. For example,circuit 100 may be used to detect when the voltage at the output of DUT102 transitions to above or below a threshold value. As another example,the output of DUT 102 may be a square wave clock signal. Since each riseand fall of the square wave can be considered a transition, by detectingthe rise and fall, the output of device 100 at the drain terminal isessentially a detection of each rise and fall or a double of the clocksignal at the DUT.

The physics of modern electrical devices and the methods of theirproduction are not absolutes, but rather statistical efforts to producea desired device and/or result. Even with the utmost of attention beingpaid to repeatability of processes, the cleanliness of manufacturingfacilities, the purity of starting and processing materials, and soforth, variations and imperfections result. Indeed, such variations maybe required to accommodate for other variations in the device material,dimensions, or method of making that device. Accordingly, no limitationin the description of the present disclosure or its claims can or shouldbe read as absolute. The limitations of the claims are intended todefine the boundaries of the present disclosure, up to and includingthose limitations. To further highlight this, the term “substantially”may occasionally be used herein in association with a claim limitation(although consideration for variations and imperfections is notrestricted to only those limitations used with that term). While asdifficult to precisely define as the limitations of the presentdisclosure themselves, we intend that this term be interpreted as “to alarge extent”, “as nearly as practicable”, “within technicallimitations”, and the like.

Therefore, the foregoing description provides those of ordinary skill inthe art with a convenient guide for implementation of the disclosure,and contemplates that various changes in the functions and arrangementsof the described embodiments may be made without departing from thespirit and scope of the disclosure defined by the claims thereto.

1. In a thin film field effect transistor in which a channel may beestablished in a semiconductor layer by application of a bias to a gateterminal in order to control current flow between source and drainterminals, the semiconductor layer being of a disordered material type,an improvement comprising: a shielding semiconductor layer and insulatorlayer, each disposed between the gate and the semiconductor layer inwhich the channel is established; and a first carrier injection terminalformed within said shielding semiconductor layer.
 2. The improvement ofclaim 1, in which both the semiconductor layer in which the channel maybe formed and said shielding semiconductor layer are formed of amaterial selected from the group consisting of: amorphous silicon(a-Si:H), and disordered organic semiconductors.
 3. The improvement ofclaim 1, further comprising a second carrier injection terminal formedin a same layer as and adjacent said first carrier injection terminal.4. The improvement of claim 1, wherein said semiconductor layer in whichthe channel may be formed has a thickness between 1.5 and 2.5 times thethickness of said shielding semiconductor layer.
 5. A field effecttransistor, comprising: a substrate; a gate terminal formed over atleast a portion of said substrate; a first insulator layer formed oversaid gate terminal and a portion of said substrate; a first carrierinjection terminal formed over at least a portion of said firstinsulator layer; a first semiconductor layer formed over said firstcarrier injection terminal and a portion of said first insulator layer,said first semiconductor layer being formed of a disordered material; asecond insulator layer formed over said first semiconductor layer; asource terminal formed over said second insulator; a drain terminalformed adjacent to and spaced apart from said source terminal and oversaid second insulator; and a second semiconductor layer formed over saidsource terminal, said drain terminal, and a portion of said secondinsulator layer, said second semiconductor layer being formed of adisordered material.
 6. The field effect transistor of claim 5, in whichboth the first and second semiconductor layers are formed of a materialselected from the group consisting of: amorphous silicon (a-Si:H), anddisordered organic semiconductors.
 7. The field effect transistor ofclaim 5, further comprising a second carrier injection terminal formedin a same layer as and adjacent said first carrier injection terminal.8. The field effect transistor of claim 5, wherein said secondsemiconductor layer has a thickness which is between 1.5 and 2.5 timesthe thickness of said first semiconductor layer.
 9. A field effecttransistor, comprising: a substrate; a gate terminal formed over atleast a portion of said substrate; a first insulator layer formed oversaid gate terminal and a portion of said substrate; a first carrierinjection terminal formed over at least a portion of said firstinsulator layer; a first semiconductor layer formed over said firstcarrier injection terminal and a portion of said first insulator layer,said first semiconductor layer being formed of a disordered material; asecond insulator layer formed over said first semiconductor layer; asecond semiconductor layer formed over said second insulator layer, saidsecond semiconductor layer being formed of a disordered material; and asource terminal formed over a portion of said second semiconductorlayer; a drain terminal formed adjacent to and spaced apart from saidsource terminal and over a portion of said second semiconductor layer.10. The field effect transistor of claim 9, in which both the first andsecond semiconductor layers are formed of a material selected from thegroup consisting of: amorphous silicon (a-Si:H), and disordered organicsemiconductors.
 11. The field effect transistor of claim 9, furthercomprising a second carrier injection terminal formed in a same layer asand adjacent said first carrier injection terminal.
 12. The field effecttransistor of claim 9, wherein said second semiconductor layer has athickness which is between 1.5 and 2.5 times the thickness of said firstsemiconductor layer.
 13. The field effect transistor of claim 9, furthercomprising first and second implantation regions located in said secondsemiconductor layer and substantially below said source terminal andsaid drain terminal, respectively.
 14. A field effect transistor,comprising: a substrate; a source terminal formed over said substrate; adrain terminal formed adjacent to and spaced apart from said sourceterminal and over said substrate; a first semiconductor layer formedover said source and drain terminals and over exposed portions of saidsubstrate, said first semiconductor layer being formed of a disorderedmaterial; a first insulator layer formed over said first semiconductorlayer; a first carrier injection terminal formed over at least a portionof said first insulator layer; a second semiconductor layer formed oversaid first carrier injection terminal and a portion of said firstinsulator layer, said second semiconductor layer being formed of adisordered material; a second insulator layer formed over said secondsemiconductor layer; and a gate terminal formed over at least a portionof said second insulator layer.
 15. The field effect transistor of claim14, further comprising an adhesion/insulator layer formed over saidsubstrate and under said source and drain terminals and said firstsemiconductor layer.
 16. The field effect transistor of claim 14, inwhich both the first and second semiconductor layers are formed of amaterial selected from the group consisting of: amorphous silicon(a-Si:H), and disordered organic semiconductors.
 17. The field effecttransistor of claim 14, further comprising a second carrier injectionterminal formed in a same layer as and adjacent said first carrierinjection terminal.
 18. The field effect transistor of claim 14, whereinsaid first semiconductor layer has a thickness which is substantiallyequal to between 1.5 and 2.5 times the thickness of said secondsemiconductor layer.
 19. A transition detection device comprising: athin film field effect transistor in which a channel may be establishedin a semiconductor layer by application of a bias to a gate terminal inorder to control current flow between source and drain terminals, thesemiconductor layer being of a disordered material type, the thin filmfield effect transistor further comprising: a shielding semiconductorlayer and insulator layer, each disposed between the gate and thesemiconductor layer in which the channel is established; and a firstcarrier injection terminal formed within said shielding semiconductorlayer; said carrier injection and said source terminals electricallyinterconnected to a system ground potential; said gate terminalelectrically interconnected to an output signal of a device under test;and said drain terminal connected to provide a detection signal outputfor indicating a transition of the output signal of the device undertest.
 20. The transition detection device of claim 19, wherein saidtransition is a transition either from below to above or from above tobelow a threshold voltage of the output signal of the device under test.